Wind turbines are designed to convert wind into electricity, by turning a generator positioned in a wind turbine housing, also known as a nacelle. The rotation of the generator is achieved by wind turbine blades, normally three, that rotate by the wind. In order to be able to optimize the output power of the wind turbine, the blades may be rotated around their longitudinal axis. In this way, the blades can be used to control the amount of wind power transferred from the wind to the generator.
Normally, especially larger wind turbines are designed for a specific wind speed range. Below a predefined wind speed, the rotor will not rotate at all. The maximum power output may be designed to take place at around 15 m/s. The wind turbine may be equipped with a gear box in order to optimise the rotational speed of the generator. Most wind turbines are provided with a variable-speed turbine, which use a solid-state power converter to interface to the electrical transmission system. In this way, the energy can be collected regardless of the rotational speed, i.e. frequency, of the turbine.
At higher wind speed, the output power of the turbine must be limited. The rotation of the turbine may be limited by altering the blade angle. Normally, this is referred to as either stall control or pitch control.
In stall control, the angle at which the wind strikes the blades, i.e. angle of attack, is increased, which reduces the induced drag. A fully stalled turbine blade, when stopped, has the flat side of the blade facing directly into the wind. In pitch control, the angle of attack is decreased, which reduces the induced drag. A fully furled turbine blade, when stopped, has the edge of the blade facing into the wind.
Modern turbines all pitch the blades in high winds. Since pitching requires acting against the torque on the blade, it requires some form of pitch angle control, either hydraulic or electrical. The blade is attached to the rotor using a rotational bearing of some kind.
Normally, slewing bearings are used in order to be able to rotate the blade of a wind turbine. The blades may either be rotated at the same time with the same mechanism, or they may be individually rotatable. Such a bearing may be a roller bearing having a diameter in the same order as the outer diameter of the blade base. The bearing is exposed to several loads, both static and dynamic. The bearing must be preloaded and the preload must further be monitored in order to identify deviations that require a readjustment of the preload.
The adjustment of the blade pitch can be seen as an oscillation, since the blade is moved back and forth. The blade will thus never rotate continuously around the centre axis. The pitch movement can be divided into three ranges.
In the first range, the pitch angle is small, in the range of up to 5 degrees, and the pitching speed is high. In this range, each blade is adjusted individually in order to compensate each blade for small deviations in the wind. Such a deviation may occur when the lower blade passes the tower. The blade can also be pitched in order to compensate for predicted or measured wind shear or other turbulences. In this range, the blade may be rotated over the range of 5 degrees in around a second.
In the second range, the pitch angle is somewhat larger, in the range of up to 30 degrees, and the pitching speed is slow. In this range, the blades are usually adjusted collectively in order to compensate for deviations in the wind speed. In this range, the blade may be rotated over the range of 30 degrees in around ten seconds.
In the third range, the pitch angle is large, around 90 degrees, and is used to park the rotor when the wind speed is too high. In this range, the rotational speed is the same as in the second range. This position is also used to stop the rotor when there is a failure in one or more of the turbine components, in order to prevent further damage to the components and to allow for a repair.
Since the pitch movement of the blade can be seen as an oscillation, the design of normal slewing bearings are not optimal for this type of application. Together with the different types of load imposed on the bearing when mounted to a blade in a regular way, there is room for an improved wind turbine blade pitch bearing.